Smita Srivastava

Inducible nitric oxide synthase in T cells regulates T cell death and immune memory.

The progeny of T lymphocytes responding to immunization mostly die rapidly, leaving a few long-lived survivors functioning as immune memory. Thus, control of this choice of death versus survival is critical for immune memory. There are indications that reactive radicals may be involved in this death pathway. We now show that, in mice lacking inducible nitric oxide synthase (iNOS), higher frequencies of both CD4 and CD8 memory T cells persist in response to immunization, even when iNOS(+/+) APCs are used for immunization. Postactivation T cell death by neglect is reduced in iNOS(-/-) T cells, and levels of the antiapoptotic proteins Bcl-2 and Bcl-xL are increased. Inhibitors of the iNOS-peroxynitrite pathway also enhance memory responses and block postactivation death by neglect in both mouse and human T cells. However, early primary immune responses are not enhanced, which suggests that altered survival, rather than enhanced activation, is responsible for the persistent immunity observed. Thus, in primary immune responses, iNOS in activated T cells autocrinely controls their susceptibility to death by neglect to determine the level of persisting CD4 and CD8 T cell memory, and modulation of this pathway can enhance the persistence of immune memory in response to vaccination.

Cutting Edge: Direct Recognition of Infected Cells by CD4 T Cells Is Required for Control of Intracellular Mycobacterium tuberculosis In Vivo

Effector T cells control intracellular infection by secreting cytokines and through contact-dependent cytolysis. Because cytokines can diffuse and act at a distance, we determined whether cytokine diffusion is sufficient to control Mycobacterium tuberculosis or whether direct recognition of infected cells by CD4 T cells is required. Using MHC class II (MHC II) mixed bone marrow chimeras, we compared the bacterial burdens in lung myeloid cells that were capable (MHC II+/+) or not (MHC II−/−) of being recognized by CD4 T cells. MHC II+/+ cells had lower bacterial burdens than did MHC II−/− cells. CD4 T cell depletion increased the number of bacteria associated with MHC II+/+cells but not MHC II−/− cells, indicating that direct recognition of infected cells by CD4 T cells is required for control of intracellular M. tuberculosis. These results show that the effector mechanisms required for CD4 T cell control of distinct intracellular pathogens differ and that long-range cytokine diffusion does not contribute to control of M. tuberculosis.

Beyond macrophages: the diversity of mononuclear cells in tuberculosis

Mycobacterium tuberculosis, the bacterium that causes tuberculosis (TB), is an intracellular pathogen of mononuclear phagocytes. Although M. tuberculosis has traditionally been thought to survive and replicate in macrophages, recent work in our laboratory and others has revealed that M. tuberculosis infects multiple subsets of mononuclear phagocytes in vivo and in vitro. In experimental animals, M. tuberculosis infects no fewer than five distinct cell subsets in the lungs, including resident alveolar macrophages and 4 types of cells that recruited to the lungs in response to inflammatory signals: neutrophils, monocytes, interstitial macrophages, and dendritic cells. A characteristic of the adaptive immune response in TB is that it is delayed for several weeks following infection, and we have determined that this delay is due to prolonged residence of the bacteria in lung phagocytes prior to acquisition of the bacteria by dendritic cells. Among the mechanisms used by M. tuberculosis to delay acquisition by dendritic cells is to inhibit apoptosis of alveolar macrophages and neutrophils, which sequester the bacteria and prevent their acquisition by dendritic cells in the early stages of infection. We hypothesize that each infected cell subset makes a distinct contribution to the overall biology of M. tuberculosis and allows the bacteria to evade elimination by T‐cell responses and to avoid rapid killing by antimycobacterial drugs.

Cell-to-cell transfer of M. tuberculosis antigens optimizes CD4 T cell priming.

During Mycobacterium tuberculosis and other respiratory infections, optimal T cell activation requires pathogen transport from the lung to a local draining lymph node (LN). However, the infected inflammatory monocyte-derived dendritic cells (DCs) that transport M. tuberculosis to the local lymph node are relatively inefficient at activating CD4 T cells, possibly due to bacterial inhibition of antigen presentation. We found that infected migratory DCs release M. tuberculosis antigens as soluble, unprocessed proteins for uptake and presentation by uninfected resident lymph node DCs. This transfer of bacterial proteins from migratory to local DCs results in optimal priming of antigen-specific CD4 T cells, which are essential in controlling tuberculosis. Additionally, this mechanism does not involve transfer of the whole bacterium and is distinct from apoptosis or exosome shedding. These findings reveal a mechanism that bypasses pathogen inhibition of antigen presentation by infected cells and generates CD4 T cell responses that control the infection.

Antigen Export Reduces Antigen Presentation and Limits T Cell Control of M. tuberculosis

Persistence of Mycobacterium tuberculosis results from bacterial strategies that manipulate host adaptive immune responses. Infected dendritic cells (DCs) transport M. tuberculosis to local lymph nodes but activate CD4 T cells poorly, suggesting bacterial manipulation of antigen presentation. However, M. tuberculosis antigens are also exported from infected DCs and taken up and presented by uninfected DCs, possibly overcoming this blockade of antigen presentation by infected cells. Here we show that the first stage of this antigen transfer, antigen export, benefits M. tuberculosis by diverting bacterial proteins from the antigen presentation pathway. Kinesin-2 is required for antigen export and depletion of this microtubule-based motor increases activation of antigen-specific CD4 T cells by infected cells and improves control of intracellular infection. Thus, although antigen transfer enables presentation by bystander cells, it does not compensate for reduced antigen presentation by infected cells and represents a bacterial strategy for CD4 T cell evasion.

Pentoxifylline Functions As an Adjuvant In Vivo to Enhance T Cell Immune Responses by Inhibiting Activation-Induced Death

Modalities for inducing long-lasting immune responses are essential components of vaccine design. Most currently available immunological adjuvants empirically used for this purpose cause some inflammation, limiting clinical acceptability. We show that pentoxifylline (PF), a phosphodiesterase (PDE) inhibitor in common clinical use, enhances long-term persistence of T cell responses, including protective responses to a bacterial immunogen, Salmonella typhimurium, via a cAMP-dependent protein kinase A-mediated effect on T cells if given to mice for a brief period during immunization. PF inhibits activation-mediated loss of superantigen-reactive CD4 as well as CD8 T cells in vivo without significantly affecting their activation, and inhibits activation-induced death and caspase induction in stimulated CD4 as well as CD8 T cells in vitro without preventing the induction of activation markers. Consistent with this ability to prevent activation-induced death in not only CD4 but also CD8 T cells, PF also enhances the persistence of CD8 T cell responses in vivo. Thus, specific inhibition of activation-induced T cell apoptosis transiently during immune priming is likely to enhance the persistence of CD4 and CD8 T cell responses to vaccination, and pharmacological modulators of the cAMP pathway already in clinical use can be used for this purpose as immunological adjuvants.

Mycobacterium tuberculosis EsxH inhibits ESCRT-dependent CD4(+) T-cell activation.

Mycobacterium tuberculosis (Mtb) establishes a persistent infection, despite inducing antigen-specific T-cell responses. Although T cells arrive at the site of infection, they do not provide sterilizing immunity. The molecular basis of how Mtb impairs T-cell function is not clear. Mtb has been reported to block major histocompatibility complex class II (MHC-II) antigen presentation; however, no bacterial effector or host-cell target mediating this effect has been identified. We recently found that Mtb EsxH, which is secreted by the Esx-3 type VII secretion system, directly inhibits the endosomal sorting complex required for transport (ESCRT) machinery. Here, we showed that ESCRT is required for optimal antigen processing; correspondingly, overexpression and loss-of-function studies demonstrated that EsxH inhibited the ability of macrophages and dendritic cells to activate Mtb antigen-specific CD4+ T cells. Compared with the wild-type strain, the esxH-deficient strain induced fivefold more antigen-specific CD4+ T-cell proliferation in the mediastinal lymph nodes of mice. We also found that EsxH undermined the ability of effector CD4+ T cells to recognize infected macrophages and clear Mtb. These results provide a molecular explanation for how Mtb impairs the adaptive immune response.

Apoptosis-Inducing Factor Regulates Death in Peripheral T Cells

Apoptosis-inducing factor (Aif) is a mitochondrial flavoprotein with multiple roles in apoptosis as well as in cellular respiration and redox regulation. The harlequin (Hq) mouse strain carries an aif locus modification causing reduced Aif expression. We demonstrate that activated CD4+ and CD8+ peripheral T cells from Hq mice show resistance to neglect-induced death (NID) triggered by growth factor withdrawal, but not to death induced by multiple agents that trigger DNA damage. Aif translocates to the nucleus in cells undergoing NID, and, in Hq T cell blasts, resistance to NID is associated with reduced cytosolic release of mitochondrial cytochrome c, implicating Aif in this event. In contrast, Hq T cell blasts express higher levels of CD95L, demonstrating increased susceptibility to activation-induced cell death (AICD) and apoptosis triggered by hydrogen peroxide. Superoxide scavenging protects from AICD in wild-type, but not Hq, T cell blasts, suggesting that Aif plays a crucial superoxide-scavenging role to regulate T cell AICD. Finally, the altered pattern of death susceptibility is reproduced by siRNA-mediated reduction of Aif expression in normal T cells. Thus, Aif serves nonredundant roles, both proapoptotic and antiapoptotic, in activated peripheral T cells.

A role for apoptosis-inducing factor in T cell development

The mitochondrial flavoprotein Aif facilitates murine thymocyte development by reducing oxidative stress.